1. Field of the Invention
The present invention relates to a speed control device for a DC motor, and more particularly to a speed control device which obtains a signal representative of the speed of the DC motor from a voltage corresponding to the back electromotive force generated in the DC motor during the OFF-state operation of a duty cycle control switch and which performs a feedback control based on the signal obtained.
2. Description of the Prior Art
A conventional speed control device for a DC motor is shown in FIG. 9 and mainly includes a switch element Q1 and a control pulse signal output circuit S1.
The switch element Q1 is connected in series between a DC power source A and a DC motor M, so that the power from the DC power source A is supplied to the DC motor M when the switch element Q1 is turned on while the power is not supplied to the DC motor M when the switch element Q1 is turned off.
More specifically, a power supply voltage V0 is applied to the DC motor M when the switch element Q1 is turned on. A voltage Vc corresponding to a back electromotive force is produced in the DC motor M when the switch element Q1 is turned off (see FIGS. 10(a) and 10(b)). The voltage Vc becomes higher as the rotational speed of the DC motor M increases.
A motor voltage V1 which may be the voltage V0 or the voltage Vc is applied to a voltage maintaining circuit 2 of a control pulse signal output circuit S1. The voltage maintaining circuit 2 also receives a control pulse signal V6 for controlling the switch element Q1 to be turned on or off. During a first level of the control pulse signal V6 to turn on the switch element Q1, the voltage maintaining circuit 2 outputs a voltage V2 corresponding to the motor voltage V1 which is produced when the control pulse signal V6 is changed from a second level to turn off the switch element Q1 to the first level. During the second level of the control pulse signal V6, the voltage maintaining circuit 2 outputs the voltage V2 corresponding to the motor voltage V1 at that time. Thus, as shown in FIG. 10(c), the output voltage V2 has a value substantially corresponding to the voltage Vc which corresponds to the back electromotive force produced during off-operation of the switch element Q1. Consequently, throughout the operation, the output voltage V2 becomes higher as the rotational speed of the DC motor M increases.
The output voltage V2 is subtracted from a set voltage V3 and the subtracted voltage is amplified by an amplifying circuit 4 to obtain a voltage V4 which becomes lower as the rotational speed of the DC motor M increases and which becomes higher as the rotational speed decreases. The amplified voltage V4 is compared with a non-constant voltage V5 by a comparator circuit 6. As shown in FIG. 10(d), the non-constant voltage V5 changes its value at a predetermined frequency which provides a basis of a duty cycle of the switch element Q1. The comparator circuit 6 outputs the first level of the control pulse signal V6 to turn on the switch element Q1 when the amplified voltage V4 (which becomes higher as the rotational speed of the DC motor M decreases) exceeds the non-constant voltage V5. As will be apparent from FIG. 10(d), as the rotational speed of the DC motor M decreases or the amplified voltage V4 becomes higher, the period during the amplified voltage V4 exceeds the non-constant voltage V5 becomes longer, so that the period during the switch element Q1 is kept on becomes longer. As a result of this, the duty cycle for driving the DC motor M is increased to compensate for the low speed rotation of the DC motor M or to increase the rotational speed of the DC motor M. On the other hand, if the rotational speed of the DC motor M becomes higher, the duty cycle is decreased to reduce the rotational speed of the motor M.
The construction of the control pulse signal output circuit S1 shown in FIG. 9 may be variously modified. For example, the voltage V2 of the voltage maintaining circuit 2 may be applied to a minus input terminal of the comparator circuit 6 without inversion. Further, the voltage V2 may be compared with a summing voltage of the set voltage V3 and the non-constant voltage V5.
The requirement of the control pulse signal output circuit S1 is that the comparator circuit 6 receives the voltage corresponding to the back electromotive force generated in the DC motor M directly from the DC motor or through other means when the switch element Q1 is not on; the control pulse signal V6 supplied to the switch element Q1 has a longer period of the first level for turning on the switch element Q1 as the voltage corresponding to the back electromotive force decreases; and consequently, a feedback control of the DC motor M is performed to control the rotational speed of the DC motor M at a predetermined value.
The prior art device may be properly operated as long as the output voltage V2 of the voltage maintaining circuit 2 is maintained as shown in FIG. 10(c). However, in an instant of turning the switch element Q1 from on to off, energy stored in an inductance coil of the DC motor M is emitted as a spike which changes the motor voltage V1 to a large extent as shown in FIG. 10(e).
To this end, a zener diode ZD is conventionally connected in parallel to the DC motor M to clip a spiked voltage to a predetermined value (-VZ in case of FIG. 10(e)) so as to prevent damage of the switch element Q1, etc. Thus, the spiked voltage can be reduced by determining a zener voltage VZ to have a smaller value. However, in such a case, the period of production of the spiked voltage becomes longer in response to the reduction of the zener voltage VZ.
The output voltage V2 is inputted to the amplifying circuit 4 which normally has an integration function. Therefore, the output voltage V4 of the amplifying circuit will have a saw teeth-like wave configuration which is strongly influenced by the spike as shown in FIG. 10(f). Even if the zener voltage VZ has been lowered, the output voltage V4 will still have the saw teeth-like wave configuration since the period of production of the spiked voltage becomes longer as described above.
If such output voltage V4 having saw teeth-like wave configuration is inputted to the comparator circuit 6, the output voltage V6 becomes unstable. Thus, even if the rotational speed of the DC motor M becomes low, the period during on-operation of the switch element Q1 may not always become longer but on-period of the switch element Q1 may be short at one time and may be long at the other time. To avoid this problem, the integration function of the amplifying circuit 4 may be increased to smooth the amplified voltage. However, the response speed is lowered in this case, and therefore, the rotational speed cannot be satisfactorily controlled.
Further, as shown in FIG. 10(b), the voltage Vc corresponding to the back electromotive force changes at relatively short cycle because of a contacting relationship between a brush and a commutator. Additionally, the voltage Vc may be easily influenced by various kinds of noises. Therefore, the voltage maintaining circuit 2 may not keep the voltage Vc at the level corresponding to the back electromotive force produced in the DC motor M, but the voltage maintaining circuit 2 may maintain the voltage Vc influenced by the noises as shown in the right-most portion of FIG. 10(e). Also in this case, the comparator circuit 6 may not receive the voltage Vc which corresponds to the back electromotive force.
As described above, the prior art circuit shown in FIG. 9 is easily influenced by noises, and it practically cannot perform the feedback control in a stable manner.